Geological relations in the Sesia-Val Grande Geopark are internationally renowned and of world-class scientific significance. Accessible outcrops display the effects of dramatic geological processes that shaped the continental crust at a wide range of crustal levels, from high-grade metamorphism, magmatism, anatexis and ductile deformation at depths as great as 25 to 30 km to the explosive eruption of a supervolcano at the surface of the Earth 282 million years ago. For more than 40 years, this area has served scientists as an unprecedented crustal reference section in which geophysical observations and physical processes may be interpreted in the context of geology that is observable on the ground (Fountain, 1976, Kissling 2012, and references therein). As a Geopark, this area is available to people of all backgrounds and ages to explore geological processes that molded the evolving crust of a continent, and produced the spectacular features that are preserved in outcrops that are easily accessible to most people. For example, visitors may stand on fragments of the subcontinental mantle and then walk along outcrops of clinopyroxene-rich dikes which were the result of intrusion of basaltic melts. They may also visit the contact between an enormous gabbro intrusion in the deep crust to observe granitic segregations formed by partial melting of the adjacent crustal rocks. And they may visit both the roots and the roof of a granitic pluton and marvel at the chaotic breccias produced by the explosive, caldera-forming super-eruption.
In addition to presenting to the public the world’s most accessible reference section for the continental crust, the Geopark introduces the public to processes that operate on a global scale. Outstanding examples of plate-boundary deformation and tectonics are found within the Geopark because it encompasses the Canavese Line, a 1-km thick mylonite belt that forms the westernmost segment of the Insubric Line, the major tectonic boundary separating the Austro-Alpine Domain to the north from the South-Alpine Domain (African Plate) to the south. Stacked European and African nappes (slices of rocks) which formed the Alpine belt during the collision of Europe and Africa are beautifully exposed along the lower Ossola Valley, and northwest of the Canavese Line, the public may visit exposures of high-pressure and ultra-high-pressure metamorphic rocks, and fragments of ophiolites derived from beneath the Tethys Ocean and obducted during the Alpine event. And because the Geopark extends from the plain of the River Po to the high Alps, it provides visitors with opportunities to also observe much more recent history with a record of climate change as recorded by Pleistocene geomorphology, recent glacial retreat, and patterns of human habitation dating to the Paleolithic.
The most distinguishing features of the Geopark are outcrops within the South Alpine Domain. Collectively these rocks form the Massiccio dei Laghi (Boriani et al., 1990a,b), which comprises two principal lithotectonic units, the Ivrea-Verbano Zone and the Serie dei Laghi, separated by the Cossato-Mergozzo-Brissago (CMB) and Pogallo lines. This terrane has been the object of intense and continuing scientific interest for decades because within it an association of accessible lower-, middle- and upper-crustal rocks constitutes an unprecedented model for interpreting the geophysics of the continental crust. Its scientific importance cannot be overstated, and the Massiccio dei Laghi has been the target of countless geologic field visits by universities and professional societies. The number of scientific papers referencing the Ivrea-Verbano Zone alone has increased exponentially since 1970 and now exceeds 2,500.